4-Thiapentanal (3-(methylthio)propionaldehyde, 3-methylmercaptopropionaldehyde) (MMP), is a substance which is used principally as a starting material for the chemical synthesis of the amino acid D,L-methionine or what is called methionine hydroxy analogue (2-hydroxy-4-(methylthio)butyric acid). The standard route for the preparation of MMP is the reaction between methyl mercaptan (MC) and acrolein (AC).
The basic acrolein process, which is based on the partial oxidation of propylene, is conventionally known (see Arntz, D., Fischer, A., Höpp, M., Jacobi, S., Sauer, J., Ohara, T., Sato, T., Shimizu, N., Schwind, H., Acrolein and Methacrolein, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2007, particularly pages 7-9) and consists principally of a reaction step, a quench/by-product removal step and an absorption/distillation step. In order to further purify the product (for example the removal of volatile compounds, such as acetaldehyde), one or more extra distillation steps can be employed. The partial oxidation reaction is generally conducted in a salt-cooled fixed bed reactor at operating temperatures of 300 to 400° C. The reactor is equipped with tubes filled with a particulate catalyst, and is kept at the required temperatures by circulating liquid salt which is cooled at a later stage in a heat exchanger, generally by raising of steam. The propylene and air feed streams are normally diluted with inert diluent gases such as nitrogen, steam, carbon dioxide or mixtures thereof. Hydrocarbons which do not exhibit any significant reaction on the catalyst under the normal reaction conditions, such as the saturated hydrocarbon propane, may likewise be part of the mixture. The dilution of the mixture is performed in order to moderate the peak temperatures in the catalyst bed and in order to minimize the risk of formation of explosive mixtures. The reactor is normally equipped with a post-cooling stage (the cooling medium is generally a liquid salt) in which the temperature of the gaseous mixture is lowered to 200-280° C., before it enters the quench stage of the first column. In this subsequent process step, the mixture is contacted with water in order to achieve a rapid temperature reduction. At this point, the majority of the condensation of the water produced during the oxidation reaction and of the water which is (possibly) added as a diluent to the reaction feed mixture also takes place. Downstream of the quench zone of the column, the acrolein-rich gas flows upwards in the column and comes into contact with a falling water stream, which has the task of removing unwanted by-products, mainly acrylic acid and acetic acid and other impurities. The falling water film originates from the condensation of water in the upper part of the column, which is operated at temperatures of 8 to 25° C., preferably of 10 to 20° C. Additional water streams can optionally be supplied to the upper part of the column in order to achieve a more favourable liquid-to-gas ratio. The by-products removed leave the column together with the condensed water via the bottom and are detoxified by thermal or biological means after they have passed through the stripping column, in order to reduce the acrolein content of this stream as far as possible. The acrolein is removed from the gas stream which leaves the upper part of the first column in a downstream processing stage, by absorption in a suitable medium (normally an aqueous solution). The liquid obtained at the base of the apparatus, generally an absorption column, is fed into a distillation column in which the low-boiling acrolein is separated from the high-boiling absorption medium and is recovered in liquid form. If water is used as the absorption medium, a product close to the azeotropic concentration is obtained as the top product. The main impurity present is acetaldehyde, while other reaction by-products are found in very small or trace amounts. The acrolein can be conveyed in this form to storage tanks, or can be subjected to further workup in order to increase the purity thereof by lowering the content of accompanying by-products. In view of the low oxygen content thereof, the uncondensable low-acrolein gas which leaves the absorption step through the upper part of the column can be recycled at least partly into the reactor as a source of inert material. The remaining low-acrolein gas is normally passed to an incineration unit for the disposal or detoxification thereof. The partial oxidation reaction is generally not conducted up to full conversion of the propylene feed in order to achieve the maximum possible acrolein yields. The propylene not converted in the reaction stage passes through the quench/by-product removal step and leaves the absorber via the top together with the other uncondensable gases. The use of a fraction of these low-acrolein gases as a dilution medium for the reaction mixture also achieves the additional positive effect of returning a fraction of the unconverted propylene to the reaction stage. In this way, the overall conversion of this raw material under the most suitable reaction conditions increases, which leads to a higher overall acrolein yield.
Acrolein is a very toxic, inflammable, very reactive substance having a great tendency to highly exothermic polymerization reactions. For this latter reason, a stabilizer to counteract free-radical polymerization is added in several stages in the process and prior to storage.
In order principally to reduce the safety risks in connection with the storage of acrolein, several alternative production/purification concepts have been proposed. Since the main use of acrolein is the production of MMP, the concepts generally include the reaction of acrolein to give MMP without any significant intermediate storage. For example, U.S. Pat. No. 7,531,066 describes a process similar to the above-described standard process, except that, instead of obtaining acrolein in liquid form as the top product of the distillation step, a partial condensation is conducted and the remaining gaseous acrolein is reacted directly in a further stage with liquid or gaseous methyl mercaptan in the presence of a catalyst to give MMP.
U.S. Pat. No. 5,352,837 (or WO94/29254) and U.S. Pat. No. 5,744,647 describe a process for the production of MMP, in which acrolein is first produced by the partial oxidation of propylene or propane in a reaction unit, then the reaction gases are cooled in order to remove water and by-products, and the remaining gas stream comprising principally uncondensable constituents and acrolein is contacted with liquid MMP in the downstream processing step in order to retain acrolein in the liquid phase and, in this same medium, to react the acrolein with methyl mercaptan in the presence of a catalyst to form MMP. Compared to the conventional acrolein process, the process described in U.S. Pat. No. 5,352,837 and U.S. Pat. No. 5,744,647 offers the advantage that liquid acrolein need not be isolated and stored intermediately. However, the process is characterized in that it has no partial recycling of the low-oxygen gases which leave the acrolein absorption step. The inert material required to dilute the reaction mixture before it enters the reactor is water vapor (steam) in this case. These large amounts of steam which are fed into the reactor are condensed in the quench step and leave the process together with the acid by-products (principally acrylic acid and acetic acid). Compared to the above-described conventional acrolein process, this process entails the disadvantage that it has significantly higher waste water treatment/disposal costs. Furthermore, the overall acrolein yields, based on the hydrocarbon supplied to the process, are generally lower compared to the standard process. As already mentioned, the reaction, for achievement of good acrolein yields, is not normally conducted up to full conversion of the propylene supplied. Higher propylene conversion rates than the ideal range for the catalyst used give rise to higher proportions of by-products. As already mentioned, in the conventional acrolein production process, a proportion of the unconverted propylene is recycled into the reactor step as a constituent of the low-oxygen gases for the purpose of diluting the feed gas mixture. The recycling of a fraction of the hydrocarbon supplied back to the reactor with the recycle gas stream enables operation close to the ideal single-pass conversion in order to maximize the acrolein yield, while, at the same time, the overall hydrocarbon conversion of this expensive starting material is increased compared to a single-pass unit. In other words, a shortcoming of this recycling stream is that the process described in U.S. Pat. No. 5,352,837 (or WO94/29254) and U.S. Pat. No. 5,744,647 has a lower starting material efficiency (less acrolein—or MMP—per hydrocarbon unit supplied) than the conventional acrolein production process. The use of steam as an inert gas source in the last processes described may arise from the disinclination to recycle the low-oxygen gas which leaves the absorption stage back to the reactor, since this gas contains certain amounts of sulphur compounds which would adversely affect the heterogeneous acrolein catalyst, can accumulate in the system or can form unwanted by-products, which would entail considerable disadvantages.
U.S. Pat. No. 4,225,516 or DE2627430 describes a process for the preparation of MMP, in which, according to the examples, a reaction gas containing 48.2 mol % of water, 41.6 mol % of N2, 5.55 mol % of acrolein and 0.65 mol % of acrylic acid is fed into an acrylic acid removal unit, is cooled later to about 0° C., in order to remove water, and then runs through an absorption unit in which the acrolein is absorbed in MMP. The MMP enters the upper part of the column at temperatures of about −10° C. The mixture of MMP and acrolein obtained in the bottom of the column is passed through a reactor in which the acrolein reacts with methyl mercaptan in the presence of a catalyst. In this process, methyl mercaptan is added continuously to the reactor. The gases which leave the absorption unit are fed into the incineration unit. The presence of large amounts of water in the reaction gas mixture which enters the purification stage of the process indicates that the source of inert material for the acrolein reaction is steam. Just like in the process described in U.S. Pat. No. 5,352,837 and U.S. Pat. No. 5,744,647, the large amount of water which is fed into the reactor, compared to the standard acrolein production process, leads to larger amounts of waste water and hence to higher treatment/disposal costs. Moreover, since no unconverted propylene is recycled to the reactor, the entire acrolein yield based on the hydrocarbon supplied is generally lower than in the standard process. This leads to a higher specific consumption of the hydrocarbon (propylene/propane) supplied per mole of acrolein (or MMP) obtained, which is a great disadvantage of these processes.
DE-102010064250.9 describes a process for the production of MMP, in which acrolein which is obtained by the partial oxidation of propylene in the gas phase first passes through a quench/by-product removal step, then is absorbed in MMP and reacts with free methyl mercaptan or with methyl mercaptan released from the 3-methylmercaptopropionaldehyde/methyl mercaptan hemithioacetal (MMP/MC hemithioacetal, from MC+MMP) formed as an intermediate to give MMP. This invention makes use of methyl mercaptan containing a relatively high level of impurities (dimethyl sulphide, dimethyl ether), and of a homogeneous or heterogeneous catalyst for the MMP reaction. In this process, the inert material fed into the reactor consists of a mixture of nitrogen, carbon dioxide and small amounts of steam. Compared to the processes using steam as the main source of inert gas, as described above, this process has the advantage that much smaller amounts of liquid wastewater are produced, but the disadvantage of a greater offgas stream, which likewise includes organic sulphur compounds. The amount of the offgas stream is also much higher compared to the standard acrolein process. Furthermore, since there is no recycling of propylene to the reaction step, this invention has a lower propylene utilization and hence a smaller amount of acrolein (or MMP) produced per molar unit of hydrocarbon supplied compared to the standard acrolein process.
It was therefore an object of the present invention to provide an integrated process for preparing acrolein by catalytic gas phase oxidation of propylene with oxygenous gas and further reaction of the acrolein produced with methyl mercaptan to give MMP, which has the disadvantages of the known processes only to a reduced degree, if at all.
More particularly, the process according to the invention should work in a very energy-efficient manner with maximum acrolein and MMP yields, i.e. should have minimum energy and steam consumption and minimum waste streams, but at the same time ensure preparation of acrolein in maximum yield based on the amounts of propylene used and, as a result, of MMP of maximum purity and in maximum yield.